Due to shrink of a semiconductor device, decrease of a process temperature is required in various semiconductor manufacturing processes. Therefore, it is proposed to use a microwave to activate impurities, crystallize amorphous, and form metal silicide. However, when a wafer provided with a metal layer such as an electrode layer or an interconnect layer is irradiated with the microwave, a heating target region (a region which needs to be heated by the microwave) may not be sufficiently heated. The reason is that part of the microwave is absorbed or reflected by the metal layer. Therefore, power consumption for sufficiently heating the heating target region increases. Furthermore, uneven heating occurs between a portion including an electrode or interconnect and a portion without an electrode or interconnect, so that a wafer cannot be uniformly heated.